Preparation, crystal structure, intergrowth and twinning of the compounds La10(C2)6Br6, Ce10(C2)6Br6 and Gd10(C2)6Cl6 are described. The compounds Ln10(C2)6X6 are formed by heating mixtures of LnX3, Ln und C in tantalum capsules under argon with the ratio LnX3:Ln:C = 2:4:6 for La10(C2)6Br6 at 1040 °C, 30 d (yield ca. 20 %), Ce10(C2)6Br6 at 1200 °C, 12 d, (80 %), Pr10(C2)6Br6 at 1140 °C, 5 d, (20 %) and 2:8:12 for Gd10(C2)6Cl6 at 1170 °C, 39 d, (100 %). They crystallize isotypical in a new structure type in C2/m with a = 22.809(5) Å, b = 3.9855(8) Å, c = 16.599(3) Å, β = 123.32(3)° (La10(C2)6Br6), a = 22.483(5) Å, b = 3.9253(8) Å, c = 16.375(3) Å, β = 123.15(3)° (Ce10(C2)6Br6), a = 22.360(10) Å, b = 3.895(2) Å, c = 16.269(8) Å, β = 123.44(3)° (Pr10(C2)6Br6) and a = 21.507(4) Å, b = 3.7193(7) Å, c = 15.331(3) Å, β = 123.34(3)° (Gd10(C2)6Cl6). The structure of Ln10(C2)6X6 is composed of Ln octahedra centered by (C2) groups, which are connected through common edges to form sheets which are further connected perpendicularly by chains of Ln triple octahedra to form tunnels, which contain the X atoms coordinating the triangular faces of the Ln octahedra. The compounds are semiconductors according to ionic formulations (Ln3+)10(C24–)6(X–)6 suggested by the C=C double bond assignment.